private void PrintAuto(IActiveView pActiveView)
{

    IPaper pPaper = new Paper();
    IPrinter pPrinter = new EmfPrinterClass();

    System.Drawing.Printing.PrintDocument sysPrintDocumentDocument = new System.Drawing.Printing.PrintDocument();

    pPaper.PrinterName = sysPrintDocumentDocument.PrinterSettings.PrinterName;
    pPrinter.Paper = pPaper;

    int Resolution = pPrinter.Resolution;

    double w, h;
    IEnvelope PEnvelope = pActiveView.Extent;
    w = PEnvelope.Width;
    h = PEnvelope.Height;
    double pw, ph;//纸张
    pPrinter.QueryPaperSize(out pw, out ph);
    tagRECT userRECT = pActiveView.ExportFrame;

    userRECT.left = (int)(pPrinter.PrintableBounds.XMin * Resolution);
    userRECT.top = (int)(pPrinter.PrintableBounds.YMin * Resolution);

    if ((w / h) > (pw / ph))//以宽度来调整高度
    {

        userRECT.right = userRECT.left + (int)(pPrinter.PrintableBounds.Width * Resolution);
        userRECT.bottom = userRECT.top + (int)((pPrinter.PrintableBounds.Width * Resolution) * h / w);
    }
    else
    {
        userRECT.bottom = userRECT.top + (int)(pPrinter.PrintableBounds.Height * Resolution);
        userRECT.right = userRECT.left + (int)(pPrinter.PrintableBounds.Height * Resolution * w / h);

    }

    IEnvelope pDriverBounds = new EnvelopeClass();
    pDriverBounds.PutCoords(userRECT.left, userRECT.top, userRECT.right, userRECT.bottom);

    ITrackCancel pCancel = new CancelTrackerClass();
    int hdc = pPrinter.StartPrinting(pDriverBounds, 0);

    pActiveView.Output(hdc, pPrinter.Resolution,
    ref userRECT, pActiveView.Extent, pCancel);

    pPrinter.FinishPrinting();

}

调用
PrintAuto(this.axMapControl1.ActiveView);